Ken:
I read about them either in a book or on the net. You might try a search in a library catalog. Here is a site with some info. http://www.eskimo.com/~enumclaw/Tips/Propagation/Rhododendron%20Propagation.html If you can figure out how to contact them they might be able to provide some info. He is a link to their contact page I think. http://www.eskimo.com/~enumclaw/sitemap.html
Mike

thanks mick .. i found that link ....

i was hoping someone with personal experience might chime in .... ken

Ken,
Yes, I am familiar with the Nearing frame. It makes a lot of sense to construct it by your pole barn which will provide some wind break protection in the winter. The ones I had at the nursery were constructed of concrete block (seconds available from a nearby company that made them) with cedar underplanking. They work well for propagation and seed starting, especially for seed that needs several years stratification before sprouting such as the Stewartias. The only problem I ever had was the occasional mouse that decided to set up winter housekeeping. They love to chew on the tender new roots. A frequently checked baited mouse trap usually took care of the situation.

Ken:
I just found this site and it is located in Fowlerville which is not to far from you. http://www.arrowheadalpines.com/propagation.htm
Mike

Ken:
I just hit on this site which has some plans and commentary http://users.stargate.net/~rocketmn/nearing.htm